This invention relates generally to the use of whispering-gallery (i.e., xe2x80x9cWGxe2x80x9d hereinafter) mode evanescent light waves to detect and/or identify chemical compounds and, more particularly, to the use of microspheres as optical evanescent-wave sensors for use in conjunction with molecular absorption spectroscopy.
Molecular absorption spectroscopy is an analytical method that is premised on the observation that each chemical species preferentially absorbs certain wavelengths of incident light radiation. Further, the suite of light frequencies absorbed by a compound can often be used to uniquely identify it. Thus, spectrographic analysis of an unknown sample by exposing to light of different frequencies is a well known way of ascertaining the identity of that sample.
Laser spectroscopy is a variant of molecular absorption spectroscopy in which the incident light originates from a laser. In broadest terms, laser spectroscopy may be said to exploit the interaction between laser light and matter as a means of identifying the particular material that is present. Laser light far surpasses other light sources in brightness, spectral purity, and directionality. Further, if required, laser light can be produced in extremely intense and short pulses. The use of lasers can greatly increase the resolution and sensitivity of conventional spectroscopic techniques, such as absorption spectroscopy.
Of particular interest for purposes of the instant disclosure is the topic of optical evanescent-wave sensors and their use in absorption spectroscopy. As is well known to those skilled in the art, when light is incident on a medium at an angle of incidence that is greater than the critical angle, Snell""s law suggests that all of the light will be reflected internally at that interface, i.e., total internal reflection. However, Fresnel""s equations (in concert with Maxwell""s equations) predictxe2x80x94and, in fact, it is observed in practicexe2x80x94that evanescent waves will be generated at the point of total reflection. The energy of this type of wave penetrates beyond the surface of the reflecting medium and returns to its original medium unless a second medium is introduced into the region of penetration of the evanescent wave. In other words, if another medium is brought near enough to the point where total internal reflection occurs, energy in the form of evanescent waves of the same frequency as the incident light will be transmitted to the alternative medium.
xe2x80x9cWhispering-galleryxe2x80x9d modes of light propagation are waves, with an evanescent component, that may be qualitatively described as traveling waves which move around a bent dielectric waveguide that closes upon itself (e.g., a sphere), with the energy confinement and guiding occurring by a physical mechanism not unlike total internal reflection in optical systems. These modes can have extremely low transmission losses, allowing such spheres to be used as microresonators with very high Q (i.e., quality factor), as was pointed out several years ago by Braginsky, Gorodetsky, and Ilchenko, xe2x80x9cQuality-factor and nonlinear properties of optical WG modes,xe2x80x9d Phys. Lett. A, 137, 393 (1989), the disclosure of which is incorporated herein by reference. This observation has motivated a good deal of recent work in which WG mode microresonators have been used in or considered for experiments in such diverse subject matter areas as cavity quantum electrodynamics, nonlinear optics, laser stabilization, precision measurement of small displacements, and single-molecule excitation and emission.
If molecules are brought into proximity with a microsphere in which evanescent waves are propagating, the molecules will interact with those waves and attenuate them to the extent that these molecules would absorb the same wavelength in conventional light, i.e., absorption spectroscopy. Further, the high Q of the microsphere means that even a single atom or molecule interacting with a WG mode can potentially have a significant effect on the energy of that mode. Light resonance within the microsphere causes it to be much more sensitive than a conventional integrated-optical evanescent-wave sensor, thus making it ideal for use in absorption spectroscopy. However, heretofore there has been no effective way of exploiting the many useful properties of microsphere evanescent wave sensors for use in the detection and/or identification of chemical species.
Thus, what is needed is an absorption spectroscopy system that utilizes dielectric microspheres as optical evanescent-wave sensors (microsensors). This sort of device would benefit from the high quality factor of the WG modes of these microspheres and would potentially be orders of magnitude more sensitive than ordinary evanescent-wave sensors. Such microsensors will be able to detect single atoms or molecules, with sensitivities better than one part per billion. Accordingly, it should be recognized, as was recognized by the present inventor, that there exists, and has existed for some time, a very real need for a device that exhibits the various characteristics described above.
Before proceeding to a detailed description of the present invention, however, it should be noted and remembered that the description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or preferred embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.
There is provided hereinafter a microsphere whispering-gallery mode evanescent-wave sensor for use in the detection and identification of atoms or molecules. In broadest terms, the instant invention consists of a microsphere into which WG mode light waves of a predetermined frequency have been introduced. When the microsphere is placed into contact with a sample, the WG mode evanescent light wave energy will interact with molecules of the sample and, if the wavelength of the WG mode matches an absorption band of the sample, the light energy emanating from the microsphere will be reduced. This approach can be used in a manner similar to conventional spectrographic analysis to identify a specific chemical compound. On the other hand, when no sample is presentxe2x80x94or if the sample does not absorb light energy at the resonant frequencyxe2x80x94and when the laser light frequency and microsphere resonant frequency are coincident, light will be emitted which is proportional in its power to the power of the input light. This effect may be used to establish a baseline emitted intensity at each light frequency.
According to a preferred embodiment of the instant invention, there is provided microsphere WG mode evanescent-wave sensor, wherein the frequency of the light within the microsphere is swept over a range of frequencies that encompasses a suspected absorption band of the sample. If the subject species absorbs light within the swept band, the microsphere""s evanescent light energy will be attenuated at one or more compound-specific wavelengths, which attenuation is then used to detect and identify the subject species according to established spectroscopy principles. Further, the sample concentration can be determined by a comparison between the absorption-reduced signal and the light intensity at another non-absorbing frequency.
According to another preferred embodiment, there is provided a method and apparatus for use in absorption spectroscopy wherein a microsphere is subject to strain-induced changes in its resonant frequency. Preferably, the strain is introduced by way of a piezoelectric transducer, although other arrangements are certainly possible and have been contemplated by the inventor. The resonant frequency of the microsphere is systematically swept over a range which encompasses the frequency of the incident laser light and, when the source of light energy is in tune with the resonant frequency of the microsphere, transmission of evanescent energy to the microsphere occurs most favorably. Then, if a sample is introduced proximate to the microsphere and if it tends to absorb light at the resonant frequency, a decrease in the output of the evanescent light energy from the microsphere will be noted.
According to still another preferred embodiment, there is described hereinafter an invention substantially similar to that described previously, but wherein the frequency of the laser light is systematically varied around the (possibly strain-altered) resonant frequency of the microsphere. As before, the light energy emitted from the microsphere is monitored for a decrease in amplitude that would indicate a substance which absorbs light at that particular frequency.
By way of general explanation, the high sensitivity of the instant invention is due to the long effective absorption path length provided by the WG mode""s large Q. This results in a detector that is suitable for use in, for example, trace-gas sensing. The instant microsphere detection system can rival the performance of a multipass cell and can be made part of a much more compact and rugged system. Among the many potential uses for the invention taught herein includes detection of carbon monoxide, carbon dioxide, and atmospheric trace gases such as methane and ammonia.
The foregoing has outlined in broad terms the more important features of the invention so that the detailed description that follows may be more easily understood, and so that the contribution to the art may be better appreciated. The instant invention is not to be limited in its application to the details of the construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. Rather, the invention is capable of other embodiments and of being practiced and carried out in various other ways not specifically enumerated herein. Finally, it should be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting, unless the specification specifically so limits the invention.